It is well known that lubricants are readily susceptible to decomposition and thus require the addition of various stabilizers and other additives in order to improve performance characteristics. Degradation of the lubricant is primarily due to the action of heat, mechanical stress (especially induced by shear forces) and chemical reagents (especially atmospheric oxygen). Deterioration of the lubricant results in an increase in total acidity, formation of gums, discoloration, loss of physical properties such as viscosity, loss of potency, polymerization, rancidity and/or odor.
This problem is particularly acute for triglyceride oils which tend to deteriorate easily due to their high degree of unsaturation. The oxidation proceeds via a mechanism which is initiated by the formation of a free radical and occurs rather easily in triglyceride oils due to the high content of active methylene groups adjacent to the double bonds. The overall effect is a high susceptability of the oil to oxidation, which is further complicated by contact of the oil with metals, such as iron and copper, present in the equipment or material to be lubricated. Metals act as catalysts in the oxidation process and accelerate degradation of the oil.
Accordingly, stabilizers are added to the lubricant in order to retard or eliminate degradation, thereby extending the life of said lubricant. For example, British Patent No. 900,756 relates to the stabilization of organic substrates subject to oxidative deterioration by the addition of metal deactivators. The stabilized organic substrates mentioned therein include lubricants, fats and oils.
U.S. Pat. No. 4,783,2725 is directed toward delaying the oxidative degradation of triglyceride oils by using selected free radical trapping antioxidants in moderate amounts. Further, EP 0,586,194 A1 relates to a stabilized triglyceride composition containing at least one alkyl phenol and optionally a metal deactivator selected from the group consisting of specified benzotriazoles, phosphatides, carbamates, citric acid derivatives, coupled phosphorus-containing amides and methyl acrylate derivatives. An aromatic amine may also be incorporated therein.
It has now been found that incorporating a certain combination of a metal deactivator for non-ferrous metals and a ferrous metal corrosion inhibitor in a triglyceride oil in accordance with the instant invention leads to surprisingly outstanding performance characteristics.